Explosives or blasting compositions of the aqueous gel or slurry type, commonly referred to as slurry explosives or blasting agents, have achieved wide acceptance as commercial blasting agents owing to their low cost, safety and inherent water-resistance. Aqueous slurry blasting agents, containing a continuous liquid phase and comprising generally an inorganic oxidizing salt (usually predominantly ammonium nitrate (AN)), a thickening agent for the liquid phase in which some or all of the oxidizing salt is dissolved, a fuel and/or sensitizer and, optionally, other ingredients such as gassing and cross-linking agents, have been very successful even in water-containing boreholes.
In order to insure adequate sensitivity of slurry blasting compositions (without incorporating hazardous self-explosive sensitizers such as TNT and PETN) non-explosive sensitizing materials, primarily finely divided aluminum particles such as paint-grade aluminum, are generally employed. For example, U.S. Pat. No. 3,367,805 discloses that the use of small amounts, 5% or less, of very finely divided or paint-grade-type aluminum containing a lyophobic coating such as of stearic acid provides significant sensitization to a slurry blasting composition. The sensitivity of slurry blasting compositions is commonly measured by its critical diameter (the smallest diameter in which a cylindrical charge of explosive will effectively and completely propagate a detonation wave and thus in which the explosive charge will successfully detonate). Even small amounts of paint-grade aluminum (1% by weight or less) have been found to reduce significantly the critical diameter of a given composition at a given temperature and thus to increase significantly its sensitivity.
The use of paint-grade aluminum as a sensitizer is practicably limited, however, due to its relatively high cost as compared with the other ingredients of the explosive. Thus many attempts have been made to find substitute, less expensive sensitizers. U.S. Pat. No. Re27,095 teaches that a combination of elemental sulfur (S) and sodium nitrate (SN) acts as a sensitizer in an aqueous or slurry blasting composition. More specifically, this patent teaches that a slurry blasting composition containing from about 5% to 25% by weight of an aqueous liquid, 40% to 70% of ammonium nitrate (AN), a combination of 2% to 25% SN and 1% to 8% S and additional fuel to tend to balance the excess oxygen of the oxidizer salts is significantly sensitized by the SN/S combination. This sensitization is attributed to the reaction between SN and S to form sodium sulfate, nitrogen and oxygen. This reaction proceeds more readily with SN than with AN; thus the sensitizing effect of SN/S is not observed with AN/S. This patent also teaches that SN/S provides goods sensitivity at low temperatures but does not render the composition overly sensitive and thus hazardous at higher temperatures, e.g., 5.degree. C. and 35.degree. C., respectively.
U.S. Pat. No. 3,282,752 also teaches that SN/S provides sensitization. However, this patent discloses that good sensitizing results are obtained by using the SN/S in a ratio substantially lower than stoichiometrically required for their complete reaction. U.S. Pat. No. Re27,095 discloses a preferred SN:S ratio of about 5:1 whereas this patent discloses an optimum ratio of 1.2:1. U.S. Pat. No. 3,282,752 teaches that at the lower SN:S ratios (3:1 or below) sensitization by SN/S becomes even less temperature dependent.
U.S. Pat. No. 3,473,983 also discloses the sensitizing effect of SN/S. This patent teaches, specifically, that the use of relatively high proportions of SN (generally equaling or or exceeding the proportions of AN) in combination with corresponding proportions of S sufficient to bring the SN:S ratio within limits of 4:1 to 9:1 provides a sensitive, effective blasting composition even though SN was not theretofore considered as effective an oxidizer as AN in such high amounts. Thus, an effective blasting composition is disclosed containing SN/S sensitization and SN per se in amounts up to 45% by weight of the total composition.
Although S is found to have particular sensitizing effects in combination with SN slurry blasting compositions, it has also been commonly employed as a fuel per se, either with or without SN. For example, U.S. Pat. No. 3,713,917 discloses that S can be used as an effective, solid particulate fuel in a slurry blasting composition along with and in the same manner as other fuels such as aluminum and carbonaceous materials such as gilsonite or coal. In addition, this patent discloses that the use of relatively high proportions of calcium nitrate (CN) (which theretofore had been generally regarded as a less effective oxidizer than AN in explosive compositions) provides, in combination with other specified ingredients, a highly effective and sensitive explosive composition which can be made detonable in relatively small diameters (about 3 inches or less) and one which has good water-resistance and stability characteristics. Thus, this patent suggests the possibility of a combination of relatively high amounts of CN (20% or more) as an oxidizer and S, in significant amounts, as a fuel. However, no specific mention of such combination is made nor do any of the examples disclose such combination. Thus although the prior art may suggest the possible combination of CN and S in a composition, it does not specifically disclose such combination or provide any elaboration as to the amounts and relative proportions of CN and S to be used such as are required for the present invention.
The present invention is based on the discovery that the combination of specific proportions and amounts of CN and S in an aqueous blasting composition acts as a highly effective sensitizer. In fact, although similar thereto, CN/S is surprisingly found to be a significantly better sensitizer than SN/S. And compositions containing CN/S are found to have significantly different properties than those containing SN/S.
One advantage of CN/S over SN/S is that CN/S provides a significantly higher level of sensitization than SN/S.
Another advantage of CN/S over SN/S is that CN/S sensitization is significantly and importantly less dependent upon temperature than even SN/S sensitization which, as explained above, was found to be relatively temperature independent. In particular, it is often desirable to have a composition which is sensitive and, therefore, capable of detonation in a small diameter (2 inches or less) at relatively low temperatures (5.degree. C. or lower) but yet at the same time be non-cap-sensitive at higher temperatures (20.degree. C. or higher) and thus which does not become overly sensitive at higher temperatures. It has been observed with SN/S-sensitized compositions, which are sufficiently sensitive for reliable detonation at 5.degree. C., that they in turn may become cap-sensitive at 20.degree. C. or higher and thus may become for practicable purposes undesirably sensitive. For example, cap-sensitive explosives require much greater safety precautions for handling and transportation. In contrast, CN/S-sensitized compositions can readily be made to be sufficiently sensitive for reliable small diameter detonation at 5.degree. C. but still are non-cap-sensitive at 20.degree. C. or higher and to exhibit generally a markedly less dependence of sensitivity upon temperature.
Still another advantage of the present invention over SN/S sensitization is attributable to the use of high proportions of CN. As explained in U.S. Pat. Nos. 3,660,181 and 3,713,917, CN contains water of crystallization which is released upon dissolution of the salt into an aqueous fluid solvent but which is correspondingly taken from the solvent upon subsequent precipitation of part or all of the salt. Thus, compositions of the present invention can be prepared at elevated temperatures with no or a minimum of added water or other solvent since sufficient fluidity for homogeneous mixing purposes can be provided upon dissolution of the CN and release of its water of crystallization at the elevated temperature where its solubility is high. The newly formed slurry can then be pumped or transferred into a desired container while still fluid (its temperature being above the fudge point of the salt solution). Finally, after pumping and upon subsequent cooling of the composition to the ambient temperature, part or all of the CN in solution will precipitate or crystallize thereby taking back its previously released water of crystallization and thus rendering the composition less fluid and perhaps even relatively non-fluid or hard. The transformation of a relatively fluid composition into a relatively hard one is efficacious for two reasons: (a) it renders the composition more stable by preventing migration, segregation and/or coalescense of dispersed ingredients and by making the composition more water-resistant and (b) it renders the composition relatively incompressible and thus relatively pressure independent with regard to its detonability. The end result, hardness, is the important one and thus anhydrous CN could be used since the initial solution essentially does not distinguish between released water of crystallization and added water, and the anhydrous CN will take out water of crystallization upon precipitation. However, hydrated CN is more economically available.
Compositions of the type of the present invention generally contain finely dispersed gas bubbles which lower their density and which have been found to greatly increase their sensitivity. In compositions which are not hard, this dispersion of gas bubbles allows the composition to be compressible and thus at high pressures its density may increase to such an extent that it no longer remains sensitive to detonation. If CN is so used as described above, a composition can be made which is essentially incompressible due to its relative hardness even though it contains a fine dispersion of gas bubbles. Thus, the CN/S combination of the present invention not only provides better sensitization but also, in addition, imparts desirable physical characteristics to the composition due to the water of crystallization in CN.